Go by feel. If you are doing low sets and relatively low reps rest enough so you can give it your all on the next set. I don't know any power lifters or Olympic lifters that time their rests between sets.

If you are training for endurance or even for bodybuilding you can use short rests like 30 to 45 seconds to push the endurance envelope. This of course will limit the weight you can use but it will build endurance and pump using a different energy system. Again you don't have to time it.